I remember reading an interview (I think it was on Playbill) with Chenoweth right before Wicked went into rehearsals (pre-San Francisco) where she said how excited she was to be playing Glinda because it wasn't the lead, but rather a very important major supporting character. It allowed her the freedom to really work on and explore her character without the burden of feeling like she had to carry a show. Of course, as besty said, the selling point the writers and producers decided to go with was the bond between the two witches, which meant that all publicity for the show was about Menzel and Chenoweth as a team, meaning costars.
As for Menzel's fear of being fired, it's completely understandable. Though it sold out its run, Wicked had a rough time in San Francisco. One of the problems they were facing was that Chenoweth was walking away with the show every night. I'm sure Menzel, who wasn't a part of the show for that long, was having issues making her performance memorable next to Chenoweth's, and was seeing two actors with major roles being replaced (one of them a two time Tony winner) was worried that she might be replaced as well. Luckily, the writers tweaked and improved her role and the producers gave her the time to improve her performance, because by the time I saw her in the last week of Broadway previews, she was wonderful and every bit Chenoweth's equal.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.