Board Thread:General Discussion/@comment-26109288-20160110152926/@comment-10663213-20170721133415

Only former (or sometimes future) series regulares and characters for another show.

Ex.: Caity Lotz in Arrow: she is billed as a guest star in the second and third seasons of Arrow, however, since the fourth season, she is billed as a special guest star because her character moves to Legends of Tomorrow, where she is a starring cast member. However, other actors like Brandon Routh, was always meant to neither becomes a starring cast member or having an solo series, so he was always billed as a special guest star. In the end, he joins Legends of Tomorrow as a main cast member. Actors like Emily Bett Richards, Colton Haynes, John Barrowman, Juliana Harkavy, Rick Gonzalez and Echo Kellum weren't meant to become series regulars in their first appear de, so they were all billed as guest stars before being promoted to series regulars and, in case of Haynes and Barrowman who left the show, were billed as special guest stars after they left the main cast

In OUAT's case, Andrew J. West, Alison Fernandez and Colin O'Donoghue should be billed as special guest stars because they were all promoted before their first appearance. Rebecca Mader, Sean Maguire, Michael Raymond-James, Meghan Ory, Emilie de Ravin and Eion Bailey weren't meant to become series regulars at the beginning, so they shouldn't be billed. However, after they left the show, they all should be billed as special guest stars.