However, this is far from true!
If one looks at the films he has done very recently, it was only two years back when Big B had appeared as a ghost on screen. The man responsible for doing so was Ram Gopal Varma and the film was Darna Zaroori Hai. In one of the short stories of the film, which featured Riteish Deshmukh along with Amitabh Bachchan, the latter enacted the role of a professor who spooks his student.
In this chilling 15 minutes tale, Bachchan is shown to be scared of a mysterious figure in the house. Later, in the story's climax, it turns out that the spooky figure was none other than Bachchan himself, who actually was a ghost!
In addition, Amitabh Bachchan has earlier too appeared in flicks which have been based on super natural elements. His lesser-known film titled Hum Kaun Hai, which released in 2004 was based on Hollywood flick The Others, which was one of the most popular films belonging to horror/suspense genre. However, since there was zilch promotion of Hum Kaun Hai, it failed miserably at the box office.
AKS is another film, which comes to mind when one talks about Bachchan's involvement in supernatural flicks. Directed by Rakeysh Omprakash Mehra, it didn't succeed at the box office, as it was way ahead of its time. In the film, Manoj Bajpai's 'aatma' entered Bachchan's body and turned him into evil. So, as one can see, there are quite a few films with Bachchan appearing in supernatural/horror sagas!
However, in all fairness to Bhoothnath, the film is hardly spooky as it projects a lovable relationship between a kid and the 'bhooth' of an old man. To make matters funnier, this jolly 'bhooth' also appears suited-booted and sings and dances rap numbers with the kid.