A good test of a great ensemble for me is this:
despite the situation they're in, would I like to be part of it?
sitting imagining what I would do and say to help
It's one of the reasons I love Aliens and Dog Soldiers is another example of a rag-tag bunch of survivors that you can't help but love and root for - despite the odds against them.
The dialogue between the cast is believable and at no point does it feel as if they're trying to sell the fact they're comrades (like so many films since). It's a natural chemistry that shows you these guys are a team that would fight and die for each other.
And that's part of the charm of this film - the concept is...silly...a British team of soldiers fighting Werewolves in the Scottish Highlands, but you're able to suspend your disbelief, due to the gritty interactions between the characters.
It's gory, funny, dark and entertaining and how I wish we got the sequel that was promised at the time.