Verbal noun and relative clause

Here is a question to which I fail to get answer from grammar book

On what occasion we use relative clause to update the information about the antecedent instead of using verbal noun.

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I saw a man walking.
vs
I saw a man who was walking.

These? Could you be more specific-- perhaps supply some examples?
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Yes, your examples exact meets my intention
could you explain the defference between them more

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The first is a shortened form of the second.

I saw a man (who was) walking.
A pencil (that was) lying on the table caught my eye.

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Thank you Micawber
Further questions:
In which case should a relative clause and participle be used to mortify the antecedent ?
Can either one of two forms take place of each other in every sentence?

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I cannot vouch for every situation. Offhand, in all the sentences I can think of, they are both possible.
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