Many judges (if not all of us) knew MTG as a player. Later they became judge.
Most of new judges will have some confusion of the two different roles they have in a tournament. Especially regular REL allow a judge acts as a player in the same tournament.

There are a lot of these scenarios, just take some of them:
a. You are playing a game and notice the player sit besides you attacked with a creature with vigilance and tap it.
b. You freind as a player asked you if he can active Windbrisk Heights when attach with Hero of Bladehold
c. Your oppent did not notice his trigger ability (e.g. Extort).

We have a common confusion here: how much should judge involve into a game?
Shall we act as a program like Magic Online?
Or we move to the other end, just watching until we have to be involved?

Feel free to post what you think. It should be a good subject for judge conference.

I don't think there would be a strict line between judge and player in regular REL event.

I think judge could initialtively fix mistakes, but shouldn't help too much like telling players how to win. (You could tell him/her how to play better after his game.)

I think my job in regular REL event is to make players happy and have fun, ensure them play fairly.
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And I'd like to educate players in regular event, telling them NOT dicide win or lose by dice, and let them know Cheat, Bribery is a very bad thing. And let them know MANA WEAVING is manipulating library.

I can answer case A and case B when you are both a judge and a player:
a) you notice a game rule getting broken: pause the match and fix it however its appropriate.
b) A player asks you a rules question: feel free to answer a rules question.

If your question is about when you are NOT judging but are merely playing in a rel regular event:
go up to the judge of the event prior to its start. Inform him or her that some players may wish to ask you rules questions. Ask the judge if its okay to answer simple questions or if he or she would prefer you to simply direct them to the person judging the event. Then just go along with what they say.

As for the triggered ability of extort- well its a may ability, so you aren't required to say anything under any set of rules. But if you want to, you are free to remind your own opponent.

The point is players know you as a judge and a player, they will take color glass on you.
As a player, they expect judge to be fair. Sometime you just to be fair, but not the way players expected.
It's very hard for new LV1 to handle the balance.

I don't mean judge cannot play as a player, however it's the matter of role and responsability.

Well, I think when you observe anything wrong, you should point out no matter you are a judge, a player or even just a spectator. So, just like Ji Li and William said, “pause the match and fix it”.

About “missed trigger”, extort, and such, is optional, it may not a good idea to point it out during the game, point out to player after the game is a good idea. (remember to tell them as it is optional that is the reason why you cannot point out during the game)

For non-optional one, it is simply an error that we would simply point it out. I think just act as what you would do if at the judge role would be good enough.