The Walking Dead made a surprising reveal about Carol (Melissa McBride) in the midseason 10 premiere episode, but it was actually set up all the way back in season 1. Apparently, Carol is claustrophobic. Carol's issues with being in confined spaces came up when she and the others were trapped in the cave with Alpha's horde.
In The Walking Dead midseason 10 finale, Carol led Daryl (Norman Reedus), Jerry (Cooper Andrews), Aaron (Ross Marquand), Kelly (Angel Theory), Connie (Lauren Ridloff), and Magna (Nadia Hiker) straight into a trap set up by Alpha (Samantha Morton). The group found themselves stuck in a cave that occupied Alpha's zombie horde. The midseason premiere deals with the characters working together to escape the cave. During this time, Carol is forced to deal with her claustrophobia and experiences an episode where she gets visibly upset.
Prior to Carol's outburst, Daryl warned everyone in the group that Carol was claustrophobic. This comes as a surprise to fans, who haven't seen Carol deal with this problem before. But apparently, Daryl - who never found this out from Carol - discovered this on his own. Audiences, however, may not have been aware, but there was a hint of this in a previous episode. In The Walking Dead season 1 finale, Rick's group are at the CDC when Carol's concerns about them being underground lead Dr. Jenner to ask if she's claustrophobic, to which Carol replies, "A little."
Talking Dead treated the reveal as a "new character trait", and claimed that this was added due to input from McBride, who felt that Carol was claustrophobic. To some, it would be odd for it take ten seasons for this to come up, but the scene in the season 1 finale confirms that this didn't come out of nowhere. Carol was always claustrophobic; it's just that this episode was the first instance where it was a real problem for the character, who had - up until now - managed to avoid situations that would trigger her claustrophobia.
Either way, Carol having claustrophobia adds an interesting layer to a character who for the past several seasons has held a reputation as a tough, no-nonsense figure who's willing to do whatever it takes to ensure the survival of the people around her. A season 7 episode explored Carol's vulnerable side when she and Maggie (Lauren Cohan) were kidnapped by Saviors, and there have been other moments scattered across the series that display other facets of the character.
It's not clear where The Walking Dead is going with its latest arc for Carol. Sunday's episode ends with Carol in tears, begging Daryl to blame her for what happened to Connie. Carol understands that her desire to kill Alpha and avenge her son's murder has caused her to make a costly mistake, and it'll be interesting to see what Carol will do next.