The Book That Wrote Itself is a 1999 road movie starring Liam O Mochain, Antoinette Guiney and features cameo appearances from George Clooney, Kenneth Branagh, Melanie Griffith, Bryan Singer, and Chazz Palminteri among others. The film was written and directed by Irish filmmaker Liam O Mochain (W.C.(film)). The film screened at numerous film festivals worldwide and was released in 2000.

Based on the paragraph above can we conclude that "There are additional cameo appearances that remain secret to this day."?
It's impossible to say