I hope I don't still come across as a great big jerk head, but if I do, don't be afraid to tell me. I have learned from my earlier mistake and I have no qualms about being put in my place. Also, feel free to disagree and add any other observations you have.
So. Without further ado; Aggressive Male Affection
(AKA: Writing certain forms of Sam/Gene slash can be difficult. Hopefully these very subjective observations can help.)
First of all, what do I mean by 'certain forms'? Well, I'm primarily referring to longer, plotted slash pieces that aren't knowingly fluffy or crackalicious. Romances, but not necessarily of the Mills & Boon/Harlequin variety. It's a nebulous region of fiction that actually encompasses multiple genres and styles, but at its core is about an emotional & sexual relationship between Sam and Gene.
Second; This is my opinion, not me saying "this is how things must be done!" I use the imperative because I am lazy and cannot be bothered to add a hundred "you could do this"es to every line. Feel free to take what you want, discard what you don't.
1. Know your theme. Know why you're writing the story. Know what you want to attempt to concentrate on. And if your story doesn't follow your intended theme, change the theme to the one it does follow and edit. I've written a few stories where I started out thinking I was writing 'that', when I was really writing 'this'. I didn't go back and modify the indicators pointing towards 'that'. I ended up with a bit of a mess.
2. Sam and Gene are awesome characters to write. Sometimes they're awesomely hard to write in these kinds of stories because they can both be absolute bastards and you want to change them significantly in order for your story to work. I suggest only doing this if that's the point. In the meantime;
a) Getting the way they talk and especially, talk to each other, helps. Listen to the show, read transcripts, read other fic writers who you think have them down well.
b) Be observant of character quirks, mannerisms and traits, but don't overuse them. We know Sam's anal (hur hur), so think about adding in the small observation of him straightening his knife and fork as he sits down at the canteen table. The use of these sorts of details can sometimes be a very subtle indicator of mood or progression. They can sometimes be akin to a sledgehammer. Chances are (though not always), your readers know who the characters are, but building these up adds to your characterisation.
c) Don't be afraid to have Sam and Gene in conflict with each other. Lots of people are reading Sam/Gene slash as much because of the problems as despite them. I think one of the reasons Sam and Gene are hot is that they punch each other. Not everyone does, but I can't be the only one, can I?
3. a) Remember that there's just as much conflict within Gene as he has with Sam. The same sometimes goes doubly for Sam himself. For a great deal of Life on Mars, Sam doesn't even really think that Gene is real (although, importantly, he acts like he does.)
b) Sam doesn't cry all the time. Gene doesn't only use violence. They are layered. In fact, they're so layered it's occasionally difficult to balance them. Therefore;
c) Don't try and cram everything about a character into one story if it doesn't fit. If you don't want to deal with Sam's coma/insanity - don't. If you don't want to deal with Gene's corruption - don't. Save them for other stories. It's very hard to get everything about a character into a story that's already dealing with a set theme and it can become overwhelming. Don't feel like you're stripping characters of their dimensions, you're not --- you're focussing on certain aspects for good reasons.
d) Sam and Gene really don't fall into dominant/submissive roles all that easily. If you want them to, you should acknowledge this. What got them to this point? Is it really about dominance and submission? Perhaps Sam and Gene's ideas of dominance vary?
4. Light and shade. Humour often highlights the drama. Life on Mars is always a very funny show, even at its most dark. Gene sometimes uses humour as a defence, and sometimes is just funny. Sam has a dry wit. It can be jarring so you have to be careful, but sometimes a well-placed joke can really round out an angsty/subdued story and vice versa; touch on something serious in humour if you feel you need to.
5. If you're anything like me and you don't really want the characters declaring that they love each other, but they so obviously do, tell the truth with a lie. My favourite way of saying that Sam and Gene love each other is to say they don't. This works best if you're using a tight third person or first person point of view, where you're getting into a character's thoughts. Sam and Gene are gifted when it comes to denial.
However, if you do want the characters declaring that they love each other, get them to a point when they would. Show (in longer 'first time' stories) or allude to (in 'established' or shorter fics) the progression that got them to that point. Otherwise, you do tend to run the risk of a reader thinking it's wildly out of character. Then again, some people won't care. It depends upon the effect you're going for.
6. The things they don't say are as loud as the things they do. Everyone knows by now my favourite scene of Life on Mars. It's the epilogue to 2.02, when Gene says 'thank you' to Sam. Two words and yet they mean so much. There's a whole unspoken speech there about camaraderie and trust and appreciation. And I love it, I love it to death. Sometimes, it's the absence of a conversation that makes the meaning. And as in this example, it doesn't have to be negative. But it does work especially well in creating relationship angst, if that's what you're after.
7. Actions are important. Sam and Gene are both men of action. This isn't always a good thing. It isn't always a bad thing. You can twist this to your advantage in pretty much any situation. It's tangled up with "show, don't tell". Plus, sometimes characters say one thing and do another.
This one, which I am still learning myself. In fact, the one I struggle with all the time;
8. Sometimes, you have to let go of the mentality that "canon is the be-all and end-all." Tell a story. As long as you are internally consistent and justify yourself in the text, you can take characters on all sorts of paths that, realistically, would never have been taken by canon. And that's what fan fiction is about for lots of people. It's extrapolation. If this happened, and this was the context, what would we end up with? How would the characters react? I'm not saying you suddenly change the characters to suit your needs (don't like that Gene's blond, make him a red-head.) It isn't about that. It's about building it up, convincing the reader. It can be done with a line here, a scene there --- exposition is not evil, so if you need it, use it. Show as much as possible through actions and interaction.
And finally; don't be like me and spend hours upon hours being neurotic about this. Write it because you want to, because you love writing, because you love Sam and Gene --- and importantly, Sam and Gene together. Enjoy yourself! Challenge yourself! And, of course, post your fics so other people can enjoy them too.