Welcome to Librarium Online!
I just got Dark Heresy and after looking through the book a bit I don't understand how characters get shoehorned into certain roles. For example, I want to play a character with the smarts of an adept who can still use swords and the like. Since stats don't go up by themselves over time it appears that as long as I role a character with decent melee stats I can play him that way. From what people have said it doesn't sound as if that is true but I don't understand why that is. Could somebody explain to me exactly how the character advancement in DH make certain characters better at certain sorts of combat?
Adepts pay top xp for improving their weapon skill, a total of 4750 for the full +20 increase available to them. They also lack automatic access to more than primative melee weapon training until rank 4 and have no real access to the melee talents ever- forcing them again to spend high xp costs and in-game effort to acquire them.
The assassin on the other hand can get their +20 increase for a total of 1600 and have multiple melee talents available from as early as rank 2.
This makes it much easier for the assassin to advance as a close combatant - while a high rolling adept may have greater raw potential they will soon fall behind an average assassin in ability and would have to pay through the nose to catch up - after all an assassin can boost their WS by 10 right out of the gate with starting xp and have points to spare, while the adept can't buy upgrade 1.
If you want to play an intelligent character who can fight almost as good as a CC focused character, consider the Priest rather than Adept. Adepts are loremasters and advisors, NOT combat characters. Hence the massive XP cost for WS upgrades.
The raw truth is that you CAN'T have a character who is good at stuff outside their main focus without utterly gimping yourself elsewhere. Paying the extra 3150xp for an adept to match an assassin in CC means that effectively your first 4-5 ranks would be dedicated towards CC advancement, with nothing to spare for adept skills and talents.