No different than the voice actors really. I mean it'd be stupid to hire someone like Sheen if you don't have him play a major character, right?
Bethesda, Patrick Stewart, and Liam Neeson beg to differ.
Actually that supports my argument. If those two had played some While neither of those characters are in the game for long, they're major introductory characters that appear right at the forefront--just like Mansell's music--and drive the central plot forward. Also, if you think about it, Sheen as TIM has what? Half a dozen scenes spread across two games? Sure, more than your two examples, but then that's has more to do with the difference between an open world RPG and a one with an essentially linear narrative.
It's really a combination of two factors; how much the person costs to hire and where they'd have the most impact. Oblivion & Fallout 3 don't really have major antagonists or really important protagonists and you can pretty much ignore the central plot and go do whatever you want, hence all the expensive talent being front loaded in the tutorial levels. The Mass Effect games ahve a more traditional narrative structure so you can have "big name" actors take small but important roles and scatter their appearances throughout.