Spike was one of the most popular characters in the series Buffy the Vampire Slayer, but the original plan was to bring him in as the Big Bad for a few episodes and not bring him back, but viewer reaction was strong enough they wanted to use him again and he was brought back, still as a Big Bad.
When he came back in, during the 4th season, he was given an implant that would not allow him to hurt others. While this dramatically changed Spike as a character and led to a story line that lasted until the end of the series, it also led to a time when Spike was nothing more than weak comic relief. (This included scenes like the one in the graveyard where he was measuring the size of a crypt so he could fit his furniture in it.)
Joss Whedon is well known for planning his story lines out years in advance and this one just completely stalled for a number of episodes, during which we saw a strong willed character turn into quite a wimp, before he finally decided what he wanted to do.
Has Whedon (or anyone else involved in the production of Buffy) made any comments about the difficulty of handing this story arc, or if they had made changes that ended up leaving that story stalled for a while?
It just seemed so out of place and incongruent with the rest of the series to see such a strong willed character turn into comic relief that I've always wondered if there was a reason behind what happened.