You can use the following function to accomplish that
(defun execute-with-helm (command)
;; We call `unwind-protect' to ensure that `helm-mode' is
;; disabled even though `command' doesn't complete normally.
;; Without `unwind-protect', if the user presses =C-g= ...
I'm on Spacemacs & am not sure whether this is specific or "universal", but checked at Helm Do Ag's Help:
Previewing an item (search result line):
C-S-j/C-S-down helm-follow-action-forward -- select next line and preview
C-S-k/C-S-up helm-follow-action-backward -- select previous line and preview
C-i <not stated in help> -- preview ...
It is called helm-follow-mode.
According to Helm documentation (C-h m from inside an Helm buffer):
When helm-follow-mode is on (C-c C-f
to toggle it), moving up and down Helm session or updating the
list of candidates will automatically execute the
persistent-action as specified for the current source.
If helm-follow-mode-persistent is non-nil, the state ...