All 3 Uses

earnest
in
The Wedding, by Nicholas Sparks
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I sat in silence as the negotiations began in earnest.†
Chpt 3 *

in earnest = serious; or in a serious manner
It might have been the seriousness with which I spoke, or the earnestness of my gaze, but as we stood together, she looked up and seemed to study me.†
Chpt 6

earnestness = sincerity or seriousness
The river was gray and ominous; a moment later, there was a flash of light followed by the crackling of thunder, and the downpour began in earnest.†
Chpt 13

in earnest = serious; or in a serious manner

Definitions:

(1) (earnest)
characterized by sincere belief

or:

intensely or excessively serious or determined
(2) (meaning too rare to warrant focus)
Earnest can also be used as a name (variant spelling of Ernest), or to signify the seriousness of a pledge made (as when earnest money is included with an offer to purchase a home).