The question of who was supposed to play Leonard and Penny gets some of the fans that ask about it confused, people always talk about Macaulay Culkin playing these roles. I think that is kind of true but it is not the whole story. The whole thing is more complicated than that.
Johnny Galecki was supposed to play the role of Sheldon. That is what the people in charge wanted to happen at first. Then Johnny Galecki himself said no he would actually want to play the role of Leonard. So there was not really a person who was supposed to play Leonard before Johnny Galecki because the role of Leonard changed before anyone was given the part. Jim Parsons tried out for the role of Sheldon. Chuck Lorre was not sure about him. Chuck Lorre thought that Jim Parsons would not be able to perform well as he did during the audition which is kind of funny when you think about what actually happened with Jim Parsons and the role of Sheldon. Johnny Galecki played the role of Leonard and Jim Parsons played the role of Sheldon.
John Ross Bowie tried out for the role of Leonard two times. He still did not get the part. He ended up playing the role of Barry Kripke. So when people say John Ross Bowie was the choice for Leonard that does not feel right. The truth is John Ross Bowie was someone who tried out for Leonard multiple times and he failed to get the role.
Macaulay Culkin never even showed up to the audition. The people in charge asked Macaulay Culkin to come in times because the writers really wanted Macaulay Culkin to be there. Macaulay Culkin did not come to audition at all. It does not matter what Macaulay Culkin wanted to do if Macaulay Culkin does not actually show up.
The Penny situation is really bad. Amanda Walsh was in the pilot as Katie. Katie was the person who was supposed to be the one that the main character liked. Then they just got rid of that character completely.
There were a lot of people they thought about for the part like Elizabeth Berkley, Marisa Tomei and Tara Reid.
Jodi Lyn O'Keefe actually got the part of Katie at first she even came in for the table read, then they fired her right away. It was not her fault they just decided that the Penny character needed to be more serious or the Katie character needed to be darker.
Then they got rid of Katie and made a new character called Penny. They gave the role of Penny to Kaley Cuoco. The reason for this was that Kaley Cuoco is a lively person. This decision seems to go against what they said they wanted which was someone who could play a character with a bit of an edge. I guess that is just how it went. Penny is a character that Kaley Cuoco played. She did a great job of making Penny a lively character.
I do not get why O'Keefe got let go quickly. They did one table read and that was it. The change to the character makes sense. Maybe the people in charge got really nervous.
None of this follows a clean narrative, which is probably why people default to Culkin was supposed to be Leonard because it's simpler than admitting the casting was indecisive for months.