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Get everything out in the first draft. Let him ramble on all you like. Put the first draft aside for a month or so. Go back and re-read, and be absolutely ruthless in your culling when re-reading ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/23893 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/23893 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Get everything out in the first draft. Let him ramble on all you like. Put the first draft aside for a month or so. Go back and re-read, and be absolutely ruthless in your culling when re-reading his rambling. If you still can't tell if he's talking too much, hand the ms off to a good beta reader with the explicit instruction that you need to know if/when Mr. Motormouth is overtaking the narrative.