I Know What You Did Last Summer:
Sarah Michelle Gellar [Helen] and Ryan Phillippe [Barry] both went on to co-star in Cruel Intentions which was produced by I Know and I Still Know co-producer Neil H. Moritz.

As well as this, Sarah Michelle Gellar also had a small role in Scream 2 which was scribed by I Know...'s writer Kevin Williamson. She died in both movies...

Jennifer Love Hewitt [Julie James] has a recurring, starring role in the TV series Party of Five, which she co-stars with Neve Campbell - who is the star of the Scream films, which are penned by Kevin Williamson.

I Still Know What You Did Last Summer:
After the credits are over, there is the sound of a hook slashing through flesh and Ben Willis' horrific (distorted) laugh -- could this mean that Julie really was a gonner?

Jennifer Love Hewitt's stunt double in the movie was also the double for Sarah Michelle Gellar in the original "I Know What You Did Last Summer"