Mr. Natwarlal (1979)
When the film started Amitabh had become the numero uno actor of Bollywood. And he was paired with Rekha. Music by Rajesh Roshan the songs were recorded and shooting commenced. Except that there was no script. They just hand a few scenes in their mind that sounded exciting but nothing concrete on paper. But the producers didn't wanted to loose the dates of Amitabh so they began shooting without a script. So they shot the songs first and by the time they shot the songs they put together scenes to go with the songs and that's how the film was made where the script came after the songs and was made in a improv rather than bound script. The film was inspired by a real life thug but was made as a comic masala movie. Even then the film was a major commercial success.
Ugly (2014)
The script or Ugly was written but never shared with the actors. The film involved a lot of improvisation and actors were supposed to interpret the scene on their own. The actors were given plot outlines and character sketches but not a proper script. One of the most powerful scene of the film is in the police station where the protagonists and police inspector engage in a dialogue over a mobile phone rather than looking for the lost girl, was completely improvised. The whole film was shot in a way that the actors themselves were suprised when the story got a twist or suspense is revealed. It helped the director Anurag Kashyap get real emotions out of the actors rather than rehearsed ones. And Ugly is considered one of the best films of Anurag Kashyap as a director.
Hamshakals (2015)
If there is any hall of fame for bad directors Sajid Khan deserve a place in that for making the abysmal films Himmatwala and Hamshakals. Hamshakals was so bad that the actors working on it denounced it. Bipasha Basu and Saif Ali Khan outrightly told the pathetic way the movie was shot without any script. For the heroes the one liner way- Do as I keep saying and for heroines- just look good. Actor Saif Ali Khan has said that the film was based on the director's imagination, as it "didn't have a script as such". The result was cringe and offensive script and puke worthy dialogues that made the film one of the worst of all times.