I'm joining in on the second session of Only War with my friends, and I want to make a Sergeant because I'm interested in the sweeping orders mechanic. First of all, there's a Commissar already playing; do the Sergeant's and Comimissar's duties overlap too much? I assume the main benefits of being a Sergeant are a companion and sweeping orders, but I admit the aptitudes they have are a big confusing. I thought I'm a leader that's supposed to be buffing people with Inspire and orders from behind the main line, yet it seems like I'm also supposed to be swinging a chainsword around in the thick of melee combat. What are good ways to build a Sergeant to deal with these dual roles? Or, should I just focus on one? Just as importantly, are there any "traps" I should watch out for in abilities that look good but don't actually work as they seem to?

You can think of him like someone who has to take the important strategic decisions in the squad. The role of the Comissar is more towards motivations and law enforcement.

If both of them are also melee focused may be because they are not normally taking part in the fight, they only pick up their weapons when the enemy is too close to do anything else than attack it with whatever you have at hand...

Of course, this depends entirely on the type of regiment and character that you want to play.

Also, remember that if your regiment has a lascarbine or lasgun as standard equipment, your sargeant may also use and carry these weapons.

I think that the Sargeant is a type of character much more interesting for its roleplaying possibilities than for its mechanical benefits...

I think someone else on the forum described it pretty well: The sergeant is the one who orders his squad to take the hill, the commissar makes sure the squad does it.

The commissar can't pull rank on anyone, they ensure morale is high and act as advisors. And execute officers who show gross incompetence and soldiers who refuse to follow orders. Most is moral though, playing cards in the evening before battle and then shooting one of them the next day because they try to run away.