What role did Saul play when Stephen was stoned to death?

The pertinent scripture in Acts of the Apostles, chapters 6-8, isn't extremely specific. Saul was clearly a consenting observer at the very least.

Acts 7:58 - and they cast him out of the city and stoned him. And the witnesses laid down their clothes at the feet of a young man named Saul.

This is the very first mention of Saul. The witnesses essentially asked Saul to "keep an eye on their clothes" as they shed their outer garments, implying that Saul himself intended only to observe rather than participate. Stephen's "trial" had no official verdict, of course, since the reaction of the crowd precluded any deliberation or casting of ballots. Had a vote been taken, Acts 8:1 indicates that Saul would have voted in favor of the stoning.

Later, Saul (by then known as Paul) made this admission:

Acts 22:20 - 'And when the blood of Your martyr Stephen was shed, I also was standing by consenting to his death, and guarding the clothes of those who were killing him.'