Production on Castle Season 6 is currently being delayed due to an ongoing feud between ABC Studios and series star Nathan Fillion, who failed to show up to work on Friday, July 12.

Fillion is pushing for a four-day work week as part of a contract dispute.

The actor was back on set Tuesday and was scheduled to be there moving forward, so it's unclear how long production will be affected or whether similar walkouts can be expected.

While sources say that ABC and the show's producers want to keep Nathan Fillion happy, it's hard for production to move to a four-day work week, since he is in almost every scene.

He hasn't taken the feud public, however.

Thursday, he Tweeted about working with new guest star Lisa Edelstein. Then, asked by a fan if he was happy to be back with the Castle family, Fillion posted a photo of crew members.

"Wouldn't you be?" he said.

Castle is still set to return Monday, September 23 ... four-day weeks or not.

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.