Many people know that actor Eric Stoltz, originally cast as Marty McFly in the film "Back to the Future" was replaced by Michael J. Fox after only four weeks of shooting. According to recently uncovered documents, however, that wasn't the only change that was made.
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INT. DOC BROWN'S HOUSE – NIGHT

SUPER – 1955

Doc: But how did you get here?

Marty: I came from the future in the time machine you built, Doc.

Doc: Great scott! The flux capacitor. It works! I knew it! It works Marty!

Marty: Yeah, that's great.

Doc: Well, we've got to get enough fuel to get this old De Lorean powered up. So we can send you back to the future!

Marty: Yeah, actually I don't really think that'll be necessary.

Doc: What?

Marty: I don't know. I think I'm just gonna stick around here in 1955.

Doc: I don't follow.

Marty: Y'know the people are friendly, everything's cheaper. Lorraine's got a huge crush on yours truly. I'm thinking of banging that broad six ways to Sunday.

Doc: Marty! That's your mother! Show some respect for goodness' sake!

Marty: Woah relax Doc. I'm not thinkin' of marrying the broad or anything. Just want my fling with the hot thing next door you know?

Doc: Uh…yeah I really don't know if…

Marty: Hey, they don't make condoms in 1955 right? 'Cause I'm thinking of going out in the rain without a coat if you know what I'm saying.

Doc: Well, Marty I think…

Marty: Don't get your panties in a bunch Doc. I know she's clean. Hell I know that birth canal better than…

Doc: Really Marty. This talk is getting wildly inappropriate. Why don't we focus instead on how to attain the 1.21 jiggawatts needed to send you back to 1985? You really wouldn't want to alter the future by having any type of contact with your mother or father.

Marty: Yeah, how about this? I don't take sex advice from a guy who spends all his spare time with his dog, who by the way is named after another scientist who is a dude. And you don't keep bugging me about the science stuff OK?

Doc: Marty why are you being like this?

Marty: Seriously Doc, have you ever been with a woman?

Doc: I don't understand why my personal life is any of your…

Marty: Yeah, that's what I thought. Listen I'm gonna go have sex with my future mother. If you need anything just send me a telegram OK?

Marty exits the house leaving Doc Brown and his dog alone in the living room.

Doc: I really don't know what's gotten into that kid.

Doc stands in front of his grandfather clock in deep thought. He looks out the window suspiciously and pulls down the shades.

Doc: Well, it's just me and you Einstein. What do you say? A bottle of wine, some soft music.

Einstein the dog whimpers uncontrollably and curls up into a ball.

Doc: Fine. Be that way. We'll see if you get to eat tomorrow.

Einstein perks up at the talk of food and approaches. The dog starts to lick Doc's hand and then his face.

Doc: That's what I thought Einstein. That's what I thought.