If the bar for hardcore RP is set at calling other Space Marines "brother", then there'll be quite a few hardcore RP guilds.

I'd like it if everyone stayed in character when out of intense fighting, but I know that wont happen, so I'm instead going to search for an Ultramarines guild that do stay in character most of the time and join them.

Being in-character makes everything much more fun.
I'll be RPing like nobody's business when the game comes out.

That sounds fancy Tell me moar!

Couldn't care if people judge me, i'm gonna roleplay

I could only think of this. Seemed appropriate, just replace dance with roleplay.

Extremely appropriate , well done sir

Eternal War, not eternal meme.
If they have non combat area for people to hang around in then I daresay there will be RP within faction.
If your team sees your silly dance while you are meant to be shooting the enemy then you might get team killed or party kicked

Well I for one shall certainly be RPing as a psychotic warlord pyromaniac.

Oh, well now that I think about it, I guess I'll be RPing the warlord part...