They don't. They are riding from the battlefield where they have dealt with the traitor Macdonweald to their camp at Forres (that's actually the name of a place in Scotland 25 miles east of Inverness, not a misspelling of forest) where the king awaits them. They are accosted by three witches of whom Macbeth asks, "why upon this blasted heath you stop our way with such prophetic greeting." A heath is defined as an area of open uncultivated land, the very opposite of a forest.
Macbeth sees the ghost of the recently murdered Banquo.
No, Lady Macbeth is not aware that Macbeth has planned the death of Banquo and Fleance.
Macbeth kills him because Banquo starts to work out that Macbeth killed dDuncan
You need to specify. There are a number of scenes between Banquo and Macbeth.
The ghost of Banquo.
Macbeth kills Banquo in the play and then Banquo's ghost haunts Macbeth.
Macbeth's best friend was Banquo. They fought together in battle before Macbeth's rise to power.
Macbeth kills Banquo in the play and then Banquo's ghost haunts Macbeth.
Macbeth sees the ghost of the recently murdered Banquo.
No, Lady Macbeth is not aware that Macbeth has planned the death of Banquo and Fleance.
Macbeth's soldiers do not kill Banquo, Macbeth hires three murderers to do the job.
Macbeth kills him because Banquo starts to work out that Macbeth killed dDuncan
In Act 3 of Macbeth, Macbeth becomes suspicious that Banquo poses a threat to his throne. He worries that Banquo's descendants will inherit the crown, as the witches prophesied. This suspicion leads Macbeth to plot Banquo's murder.
*Banquo. Macbeth's best friend is Banquo, but he is also Macbeth's enemy. After the meeting with the witches, Macbeth finds out that he will become the Thane of Cawdor and the King of Scotland. Eventually, he achieves both of these prophecies. Banquo, Macbeth, and Lady Macbeth are the only people that know about the witches. Banquo becomes suspicious of Macbeth because Macbeth fulfills the prophecy after King Duncan is murdered, and Macbeth acknowledges this fact. Basically, Macbeth doesn't trust Banquo.
It is not known who kills Banquo but it is Macbeth who orders for him to be murdered.
In both cases, the ghost that appears in Macbeth is summoned by the guilt and inner turmoil of Macbeth himself. The ghost of Banquo is a manifestation of Macbeth's guilt over his role in Banquo's murder, while the ghost of Lady Macbeth represents his guilt over her death and his descent into madness.
You need to specify. There are a number of scenes between Banquo and Macbeth.