I'm using Onmund- the mage from Winterhold- as a companion, but instead of attacking enemies, he just holds a double-handed lightning spell and never releases it. Occasionally, he'll pull out a sword, but he never actually swings it-- he just switches back to the lightning spell and never fires it.

Could it be that he's trying to use a spell that he doesn't have enough majika for? Or just a bugged spell? I don't suppose there's a console command or something that could make him unlearn that spell...

I have the exact same problem with Onmund at this moment, he just refuses to cast any type of spell. I'll let you know if i find a solution.

Same thing happening to me. He was fine for the longest time, would attack perfectly. Then he just stopped. Have tried giving weapons, taking them all away, etc. but he just stands there in an attack position....doing nothing.

My companion Vorstag has the same problem. All of the sudden he stopped attacking all together. Now he just walks up to an opponent and just stands infront of them, weapons sheathed ,taking hits. Sometimes he just starts walking away aswell, going into sneak stops this. Any feedback would be greatly appreciated.

Have the same problem with any of the companions I get whether by assignment or by choosing.

Eventually they'll just start humping me with a weapon drawn instead of attacking which defeats my stealth.

Seriously Bethesda?

http://i.imgur.com/SMAV8.jpg

Yeah, just look at me and stay crouched ruining both of our stealth.

You know, now that I read this I realized my Lydia has been acting up the past few times I played.

Not in every fight , but in some I did notice she has had a tendency to stand around more often than before.
I didn't think much of it at the time but it had given me some minor pause to think 'wtf' is she doing?
I had attributed it to path finding errors in that case though.

I hope it is not some disease spreading.
Maybe some form of brain rot?
Er, script rot?

As an aside. Companions in this game are a huge letdown from just the default Fallout New Vegas ones.
For those of us that like using them the only hope is to get fully thought out and modded ones months from now.
/whine

Having the same problem with Marcurio - had him since the start of the game without a problem. Then I accidently became a vampire and he would join in with everyone else chanting "Burn it! Burn the vampire" - EXCEPT that he would still follow me around.
He wouldn't attack me but, problem is he stopped attacking everything else...even when they are attacking him!

I've since cured my vampirism, however he is still acting this way. Very disappointing after having worked so damned hard to keep him alive grrr...
If anyone knows a fix for this, please PM me. I'm thinking maybe if I let him go for a week (IRL) or longer, eventually it may just reset!? I don't know.
So far attempts to let him go and re-hire him haven't proved successful

EDIT FROM LAST NIGHT: ***********************************************************************

Just have to come back to say this morning that after letting Marcurio go and hiring someone else (Jenassa), I let her go this morning, went back to hire Marc and it seems as if the problem has now been fixed! YAYYYYYYYYYYYYYYYYYYYYYY lol
See before, when I would go to re-hire him, he was still in that mode where he would let me hire him for free because we'd only just traveled together. This time, because I worked with someone else, it had me pay the 500 fee all over again so I don't know, it must have 'reset' him somehow.

Can't say for sure what did it; all I know is that I'm happy again Good luck to everyone else...

I had the same problem with Farkas(or was it Vilkas?)
I wonder.... There's a known glitch where followers sometimes get stuck in a crawl pose after being hurt. You can reset their posture with a FUS... Maybe try hitting your follower with FUS and see if it resets their behavior?