I entered Schengen without having my passport checked at all (I tried but nobody was interested), and nobody said anything when I later checked in for an Schengen flight.
Entering the Schengen zone from outside is a totally different subject.
But entering the Schengen area e.g. from the UK it is possible that nobody looks at you. Mainly due to the fact that upon check-in in the UK your documents will be checked anyway.
For the intra Schengen flight there´s - as already mentionned - no requirement to check ID besides verification of the passenger matching the name in the reservation.