Leveling Up Party Members

[burritonator]

Should I switch my party members in and out regularly so they level up evenly? Or is it OK to leave a party member on the sidelines if I'm sure I'm never going to want to use them? I don't want to get to the end of the game and discover a weak character is required in my party.

[Ilja]

Party members remaining in the camp will level up when you do.

Active companions will gather experience when adventuring and might have higher XP amounts than the ones in the camp, but that does not change their level cap.

[mudfarmer]

Key plot choices you make will effect the favorability rating of the party members that are with you at the time, and diferent combinations of characters will change the party banter dialogs that happen randomly. So apart from keeping a viable mix of skills, the main reason to pick a given party combination is role-play.

[Crom]

Make your favorite group of 4. Thats how this game works the best.

They level up in camp, but, its up to you, which skills you choose for them.

[K00lex]

Originally posted by burritonator:

Should I switch my party members in and out regularly so they level up evenly? Or is it OK to leave a party member on the sidelines if I'm sure I'm never going to want to use them? I don't want to get to the end of the game and discover a weak character is required in my party.

XP is given to active companions, but the inactive ones left in Party Camp will auto-level up at one below the Warden.
http://dragonage.wikia.com/wiki/Experience#Unused_Characters

Note that when a new companion joins the group, he/she normally will be at a higher level than the Warden, so sometimes it's prudent to keep them longer in active party to continue to bump up their levels higher than the Warden. This will ease burden on combat, since challenge scale or monster levels of dungeons/areas are based off the Warden's. Plus, access to higher talents/spells.
I deactivate them upon gaining a newer companion or for roleplay.