I thought I recognized Jack Nicholson

I looked carefully throughout the film for Jack Nicholson and couldn't find him. Can you tell me what scene he is in?

Nicholson is not the gunman rubbing garlic on his bullets but is an observer. It's a scene shortly before the massacre. One of the shooters is rubbing garlic on the bullets that he is going to load in a "drum magazine" to arm his Thompson submachine gun, or "Tommy gun." It's been mentioned that some assassins did this as a precaution in case you survived the shooting, in the belief that you would die (in the days before antibiotics) from blood poisoning caused by the garlic. I have never heard medical testimony for or against this but that's what was happening in the scene.

Another gangster sees this and asks what the gunman is doing. Nicholson, sitting at the table where the preparations are being made, puts on a mock, raspy gangster's voice and says, sarcastically, "Garlic. In case the bullets don't kill you, you die of blood poisoning." Or words to that effect. If you click on the extended credits for “St. Valentine’s” on IMDB, Nicholson is identified as Gino the gunman (uncredited).

As for this being the start of something big, it was the other way around. Nicholson had been starring in cheapo B-movies made by Roger Corman since the very early sixties. Sometimes you can catch a glimpse of Jack in repeats of old TV series from that decade (although IMDB doesn't list them). I caught him in an episode of the Andy Griffith show, after Don Knotts had left and the episodes had gone to color. There wasn't anything remarkable about him, unless you had an eye for talent that was clairvoyant. According to "Easy Riders and Raging Bulls," after all those years, Jack had "hardly made a dent" in terms of achieving success and even Corman was running out of uses for him. Then, came the big break—“Easy Rider.” And you all know the rest.

On the other end of the spectrum (and to answer a user's comment), the reporter with a mustache at the end of the movie is NOT Tony Dow; he's Buck Taylor, a.k.a. Newly from Gunsmoke.

Jack was also in the scene where the whole caper is explained. He also drives the phoney police getaway car. His only line of dialog is: "Garlic. In cast the bullets don't kill you, you die of the blood poisoning."