Is there some special combination of buttons to perform the new extended assassinations in Halo: Reach? Like where you knife someone? I can only seem to do they typical Halo behind the back assassination when pushing RB.
You need to sneak up behind them and hold the melee button which is usualy the right bumper.
Yep you need to hold the button rather than press it - that's how the game decides which you do.
Assasinating someone is not always the best option, if they are weak and there are several enemies around it is best to just tap it for a regular beatdown as you can start attacking other enemies sooner. Also while you are assasinating someone you're an easy target for enemies.
They Should be unaware of you before you can make an assasination. Creep up on them and then use the melee.
Best way to try this out would be use the hologram, distract, sneak up and attack him .
There is one particular assassination I like: You can assassinate off the Spire!
This requires skill and good timing. You don't wanna kill yourself and miss. Its simple. Spot an enemy and jump down. Landing on him will kill him and should do you little to no harm!