Sven Eric Maier at 2013-07-01 13:42:53:
Buddy movies seem to work like romantic movies: Two people who are in conflict with each other end up together. Where romantic movies are about love, buddy movies are about friendship. There is the possibility of having three buddies. One is set against the other two. For example GHOSTBUSTERS, BEVERLY HILLS COP or CITY SLICKERS.
What seems to be the most important aspect of those movies is the reason why those characters are stuck together in the first place. Those people, it seems, are not made to be together. They are too different. They are opponents of each other. The reason why they share any scenes to begin with is the well-spring of their ongoing conflict, the fun for the audience and for the writer.
In cop movies like Lethal Weapon, the reason they have to work together is usually because they become partners. In Beverly Hills Cop, Axel is looking for the guy who killed his partner and the other guys are just trying to do their job. In 48 HRS. Jack gets Reggie out of prison, because he needs him to catch the criminals he's after. In DE L'AUTRE CÔTÉ DU PÉRIPH Ousmane wants to solve a murder case, so he steps outside of his boundaries and tricks the guy from homicide division into working with him. In DJANGO UNCHAINED Schultz frees Django to help him collect a bounty.
So if you try to pair a space alien and a dog-robot, you better come up with a good excuse. :D