Several sources have stated that Rick Jason was to carry the M1928A1 Thompson submachine gun. After two days of filming, Jason complained about the weight of the Thompson and switched to the lighter M1 carbine and carried it throughout the rest of the series. Vic Morrow was then given the Thompson to carry. After two weeks, he also complained of its weight. A lighter replica Thompson was made out of wood and was carried by Morrow until it was time for a firefight, at which time he would switch back to the real Thompson. The replica can be seen with its incorrect ejection port.
Vic Morrow and Rick Jason were essentially co-stars, and for this reason there were two sets of opening credits, some listing Morrow as the lead star before Jason and some listing Jason ahead of Morrow, although not necessarily based on which one had a larger role in a given episode.
Pierre Jalbert's character, "Caje", is supposed to be a Cajun (a group descended from the early French settlers of Louisiana who still speak French and keep to their own part of the state). Jalbert himself is actually a French-Canadian from Quebec.
Caje's full name and rank was Pfc. Paul Lemay, Kirby's was Pvt. William G. Kirby, Nelson's was Pvt. Billy Nelson. Littlejohn's first name was never given, and while Saunders on a few occasions was called "Chip" by soldiers who had known him in the past, including Lt. Hanley--who was in the same unit with him when both were NCOs--it was apparently a nickname and his real first name was never given. In season 1, "A Day in June", the D-Day invasion is told in flashback. Then Sgt. Hanley is addressed by a young lady with whom he was keeping company as "Gil" when the men are ordered back to their posts.
The distinctive helmet that Vic Morrow's Sgt. Saunders character wore in the series was covered with a piece of nylon cut from a U.S. paratrooper's camouflaged T-5 main parachute canopy, not a cloth Marine Corps cover as assumed by many viewers.
Beginning with Vic Morrow and as the series progressed, the principal cast members were outfitted with expensive custom-made fiberglass prop helmet shells to replace the heavier steel shell of the real two-piece M1 helmets with which they began the series.
Sgt. Saunders was referred to as "Chip" several times over the course of the series. While Chip is a common nickname for Charles, he is never referred to as Charles, Chuck or Charlie. Lt. Hanley is addressed as "Gil" in the first episode of the series. "Gil" is usually short for Gilbert, but other than that one time, Hanley's actual first name is never mentioned again.
Although Pierre Jalbert had been employed at MGM for nearly a decade, he had no actual acting experience prior to being cast as Pfc. Paul "Caje" Lamay on the series. Up to that time his job at MGM had been in the capacity of a technician in the film editing department.
The M-1 carbines carried by Lt. Hanley and various other characters have a prominent bayonet lug on the bottom of the barrel, marking them as the later Korean War variant of the weapon. The M-1 carbines issued in WW 2 were not equipped to mount a bayonet.