He must have had confidence in his speech writing ability to attempt to rewrite a speech that he was using to introduce himself to the American people. Unfortunately, it was misplaced confidence. From Novak’s column:

Fred Thompson’s admittedly disappointing debut as a potential presidential candidate addressing Republicans in Orange County, Calif., May 4 stemmed from his decision to personally rewrite an effective speech written for him.
A 1,600-word excerpt from the prepared speech, put on the Internet, energized conservative activists in Washington. But the actor-politician sat down in his Balboa Bay Club suite to rewrite the speech. Thompson does not like to read from texts, and he reduced his rewrite to notes that became the basis for his presentation.
The final version delivered was less tightly organized and more verbose than his prepared remarks. It omitted making common cause with Sen. Tom Coburn’s crusade against pork, and added a concluding anecdote about talking issues with sixth-grade students.