When you're in the gym and see this picture of available workout actions you can use the shower in the gym. If it 'X'ed' out with a
red X the shower isn't available and it won't always be available so you may have to try multiple times.
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Now you have to find Eva in the gym, preferably alone in her skimpy bikini. See those yellow and pink hearts on the achievement placeholders? They offer additional clues as to requirements like love, affection, lust, etc. If you check all those boxes, go take a shower by clicking the icon. What, she wasn't in the shower? Too bad, hope you saved before heading to the shower. But, even that doesn't guarantee you'll get the scene. It's random so it may take multiple tries.