What was the purpose of the canister shoved into the banker's mouth by the Joker?

Question

In The Dark Knight prologue, the Joker shoves a canister into the banker's mouth, and yet, the banker just sits there with the canister smoking in his mouth.

Why did he just sit there, and what did the canister do?

Answer 1

The film's screenplay and official novelisation state that the grenade spews smoke. There's no indication that it does anything other than this, nor any further mention of the Bank Manager.

[The Bank Manager’s eyes go wide. The Joker rises, strolls towards the bus, the purple thread attached to the grenade pin unravelling from the purple lining of his jacket as he walks. The Joker climbs into the bus, shuts the rear door, trapping the purple thread …]

EXT. SCHOOL, GOTHAM – DAY 17
[Kids pour out, heading onto a long line of school buses.]

INT. BANK – CONTINUOUS 18
[As the bus pulls out, the purple thread pulls the pin – Hostages scream and scurry away from the Bank Manager, who shakes with fear as, with a fizz, the grenade does not explode, but spews red smoke.]

and

The purple thread yanked the pin from the grenade in the bank manager’s mouth. Hostages screamed. The grenade hissed and began spewing red smoke, but it did not explode.

As to why the banker doesn't spit it out, the grenade is large enough that it would have locked his jaw, making it (like the proverbial nut) all but impossible to spit out, especially for someone with a chest wound.

Answer 2

If you look at the banker you can see that he has no control over his arms and legs (presumably a spinal injury from being shot).

Consequently, the only way to get rid of the cannister would be to spit it out but it appears (at least in universe) that it's shoved far enough in to negate that possibility.

As for what the cannister is, I think this is supposed to be the Joker toxin.