Let me try to define it the way I see it, from a UK perspective, which seems slightly different from the US.
It's very easy to tell, if you are in a production, who is a principal and who an extra - or as we prefer to call them over here, Supporting Artists*, SAs for short.
- A principal has a driver and car to bring them to set; an SA gets the bus.
- A principal has a trailer known as a three-way because it's divided into 3 separate accommodations - only Tom Cruise gets one all to himself**; an SA gets to share a double-decker bus or some old school/church hall [cheerfully known as 'crowd base'] and communal changing facilities.
- A Principal eats with the crew; an SA stands away from the lunch queue until everyone else has got theirs.
Note: only for Hollywood movies are the principals treated like untouchables, being fetched and carried in all aspects. For the most, they 'muck in' with the crew and don't consider themselves demigods.
- A principal is always credited, even if their character name is 'man in pub' and all they said was one line; an SA is never credited, even if they got a line. [This has some grey area - some SAs are also actors, so might in certain circumstances get a quick promotion, at the 2nd assistant director's discretion, but it is not common.]
- A principal - though not officially entitled to this perk, as it is technically only for crew, gets Craft - good coffee, snacks, smoothies, all the treats. An SA gets a tea-table - hot water, instant coffee sachets and tea bags, DIY.
To differentiate using examples from one show I know and work on, which I know also is broadcast in many countries, Call the Midwife
A Day Player is not an extra but a principal, an actor booked for a small role, which is potentially [but not always] shot in a day. Sometimes a day player can go on to become a regular character, e.g. Daniel Laurie who played Reggie in the last series, initially as a day player, but 'promoted' to the regular cast for season 7.
The day players you will see in every episode are those playing this week's 'mum & dad' - the expectant couple each episode centres on and whose story is completed in that one episode. As the shoot is 9 episodes over 6 months, that means those day players will be on set for perhaps one or two weeks.
They are Principals, named characters, and check out on all the bullet points above.
An extra, on the other hand, can be seen walking past in the background every time people are needed to make the place look busy.
They generally don't speak at all, unless some general 'hubbub' is required; they instead pretend to be having conversations, shrug shoulders, wave hands and in all tend to look slightly over-animated compared to 'real people' [in-joke]
If they are distant from the camera, this over-acting works well, if close it can look awkward.
An SA may be called upon to do a single line, specific or generic, for which they will get extra money - known as a 'feature'.
A feature is a double-edged sword. Yes, it gets you in front of the camera for your own close-up if you're lucky, but it can also lock you out of the rest of the series if production think your character would be too recognisable in other scenes.
Back to Call the Midwife... When the scene is in front of Nonatus House [where the nuns & midwives live] or in the Community Centre etc they are "in Poplar", the area of London the show is centred around.
When they are "in Poplar", all the passers-by [SAs] are 'locals' - the characters they portray all live in the area. They can therefore be the same people each time, racking up 20 or 30 days' work across the series.
If you watch carefully you will see the same people, in different outfits, walking in different couples with different kids, in pretty much every crowd scene across the whole 9 episodes. (Hence the dilemma if you get a feature - you want to swap 30 days' work for one day plus 20 quid? ;-) They do swap and change the SAs, but from a core of about 50 regulars.
When they are "not in Poplar" those same regulars cannot be seen, so a fresh batch of SAs are brought in for that.
To use another example, to show how far you can go and still be categorised as an 'extra' rather than a principal...
EastEnders. The market traders are all 'extras'. They each have their own market stall and are in pretty much every single episode - but instead of being paid by the day with extra money for being featured, they are instead contracted. They are guaranteed a given income in exchange for a set number of days' work per year. They may be called on to say lines or interact with principals at any time, without receiving specific compensation for that.
It's difficult to tell from an audience perspective. The main clue would be if they are credited - and for this let's choose to ignore edge cases like Stan Lee in the Marvel universe, also that a cameo is in itself an edge case. A famous actor doing a single line is still a famous actor and will have been given all considerations and courtesies appropriate to their status rather than role.
As mentioned elsewhere - a principal went through the audition process [though not always] whereas an SA was picked from a sheet of photos [again not always].
A principal was booked by an acting agent, an SA by an SA agency.
These are all things the audience cannot see in their attempts to differentiate.
One additional thing to note.
You cannot rely on IMDB to differentiate a principal from an extra. Many seasoned extras credit their own performances in shows, giving themselves 'popularity' and pushing themselves further up the rankings.
Their self-accreditation does not make them a principal. Only the show's official credits as rolled at the end of that show can be considered any authority on the matter.
*Yes, I'm fully aware that's like calling a spade a manual earth-moving implement, but that's their official title. The word 'extra' is frowned on in the industry [officially.]
** and Jeremy Piven [Harry Selfridge], but that's because he brought his own, from the US ;)