Final answer:
It is crucial for Macbeth and Lady Macbeth to disguise their true intentions behind a facade of innocence to successfully commit and conceal the regicide of King Duncan in Shakespeare's 'Macbeth'.
Step-by-step explanation:
In the lines "false face must hide what the false heart doth know" from Shakespeare's Macbeth, it is crucial for Macbeth and Lady Macbeth to conceal their true intentions and emotions to carry out their plan successfully. This line underscores the theme of appearance versus reality that is prevalent throughout the play. Macbeth is about to commit regicide by killing King Duncan, and both Macbeth and Lady Macbeth must appear innocent and welcoming to avoid suspicion. Lady Macbeth further emphasizes this point when she instructs Macbeth to "look like the innocent flower, But be the serpent under't" suggesting they must disguise their malicious intents with an outward show of hospitality.