On commercial flights, key checklist items are forced by using a procedure call a "cross check". The one pilot must "challenge" another for those specific check list items. For non-cross checked items, pilots have discretion to implement checklists as they see fit.
Each pilot has their own practice. Some are sloppy and do nothing. Others fastidiously follow every item. Most commercial pilots tend to blow off most checklist items, especially anything outside the cockpit. For example, I fly American a lot and when the flight crew arrives I always watch to see if they do a walkaround and I have NEVER seen them do it. I complained about this to one of the top flight instructors at American and he said, "Well, they are supposed to do the walkaround." Yeah, they are SUPPOSED to, but do not do it.
It comes down to the personal discipline and attitudes of the pilot involved and how pressured they are to get the plane up in the air and on schedule.
On non-commercial flights and cargo flights often crews will become very lax. For example, in 2014 a Gulfstream crashed at Hanscom airfield killing everyone on board because they tried to takeoff with a gust lock engaged. This is impossible if you do even a minimal checklist because a control check is like the #1 or #2 thing on any checklist. A review of the tape showed that the crew in that case did ABSOLUTELY NOTHING verbal for cross checking. Furthermore, a review of the flight recorder for the aircraft showed that the crew had NEVER done a control check in 98% of the flights in that aircraft. Such laxity is not unusual in non-commercial flights.